The present invention relates to eletrode membrane assemblies to be used in electrochemical devices, in particular in PEM fuel cells (polymeric electrolyte fuel cells), and their process.
As known, the PEM fuel cells comprise a core comprising an ionomeric membrane having on each side an electrode containing the catalyst for the combustion reaction, on each side of the membrane at least one gas diffusion layer is first placed, followed by a bipolar plate. Two sections wherein the comburent and the fuel are respectively fed, the sections being located between the ionomeric membrane and each of the two bipolar plates The comburent is generally air or pure oxygen; the fuel is for example pure hydrogen, gaseous mixtures containing hydrogen, or methanol or ethanol aqueous solutions. The two sections form a reaction cell. The key feature of a fuel cell is the Membrane Electrode Assembly or MEA placed, as said, between the bipolar plates of the reaction cell. The simplest membrane electrode assembly is formed of an ionomeric membrane, acting as electrolyte, with an electrocatalytic layer (catalyzed area) applied on both sides of the membrane. These assemblies in the prior art are known as Catalyst Coated Membrane (CCM) or 3-layer MEA.
As said, MEAs are used in electrochemical devices with at least one gas diffusion layer in contact with each electrocatalytic layer.
Other types of assemblies or MEA devices with a higher number of layers are known in the prior art. For example, the 5-layer MEA is an assembly wherein, on each of the two electrocatalytic layers of a 3-layer MEA as defined above, a gas microdiffusion layer is applied. The latter has hydrophobic characteristics, generally is a mixture of carbon powder and PTFE. The 7-layer MEA is an assembly wherein on each of the two microdiffusion layers of the 5-layer MEA a gas macrodiffusion layer is applied. The latter has hydrophobic characteristics, generally formed of PTFE-treated carbon fibers or tissues.
The single reaction cells are assembled in electrical series thus obtaining a device called fuel cell stack. The fuel cell stacks supply powers, generally between some tenths of watt and some hundreds of Kilowatt and generates heat. A cooling system is therefore necessary to remove the heat produced by the electrochemical reaction. In the stacks it is a common practice to alternate the single reaction cells with cooling cells fed with a fluid, generally demineralized water.
In the MEAs the portion of the ionomeric membrane surface coated by the electrocatalytic layer, represents a fraction generally between 40 and 90% of the total membrane surface. This surface fraction is indicated as “active area” as it is involved in the electrochemical reaction. On the remaining part of the membrane surface, i.e. on the non active area, a protective film can be applied, generally formed of an inert material towards the reaction taking place in the electrochemical device. The protective film is generally known as “subgasket” and has the purpose to improve the MEA handling, for example to facilitate the assembling in electrochemical devices and to protect the polymeric electrolyte from the contact with the bipolar plates. The obtained device is called MEA with subgasket. A 3-layer MEA with subgasket according to the prior art is illustrated in FIG. 1 in a plan view. (1′) indicates the central area (in dark) which represents the active area as above, (2′) is the membrane surface coated by the subgasket. (6′) indicates the three openings, respectively, in the upper- and in the lower-part of the MEA. When the latter is assembled in a stack, the openings form 6 distribution channels, which in couples are used, respectively, for the comburent and fuel transport and for the cooling fluid.
FIG. 2 reports a MEA sectional view along B-B of FIG. 1 (4′) represents the ionomeric membrane, the two layers (3′), positioned symmetrically with respect to the membrane, indicate the subgaskets, the two layers (5′) correspond to the catalytic layers, which coat the membrane in correspondence with the active area.
FIG. 3 reports a MEA sectional view along A-A of FIG. 1, wherein (4′) represents the ionomeric membrane, the two layers (3′), positioned symmetrically with respect to the membrane, correspond to the subgaskets. FIGS. 2 and 3 show that the subgaskets cover the non active portion of each side of the membrane. In the prior art, these electrochemical devices maintain a high efficiency for long time by using cooling fluids having a high purity degree, in order for avoiding pollution sources. Generally, when possible, in the prior art as cooling fluid, deionized water is used. Deionized water, of the quality required for the working of polymeric membrane electrochemical devices, has to be produced by a plant for the purification of water. In fact the cooling fluid amount required for the working of a fuel cell stack is quite large. Therefore, from an industrial point of view, the use of high purity cooling fluids in electrochemical devices represents an additional cost for the plant and its maintenance.
The need was felt to have available MEA assemblies with subgasket, having the following combination of properties:                possibility to use cooling fluids, for example water, having lower purity than that required by the prior art, and thus cheaper from an industrial point of view;        ability to use also cooling fluids formed of mixtures of water with high boiling solvents, for example, water/-glycols, for use at temperatures higher than 100° C., for example up to 160° C., and use at temperatures lower than 0° C., for example down to −40° C.;        durable adhesion of the subgasket to the MEA.        